Japanese Patent Publication No. 2001-74643 discloses a particle analyzer which includes a sheath flow cell for forming a sample stream by a sheath fluid and a sample containing particles such as cells or the like, a pulse light source for emitting a light pulse, optical fiber for guiding the light pulse emitted from the pulse light source, condenser lens for collecting the light pulse emitted from the pulse light source and irradiating the sample stream, and a video camera for capturing a projection image of the particles in the sample stream irradiated by the light pulse.
In this particle analyzer, coherency can be reduced to obtain particle images of small refraction fringes (interference fringes) by passing the light pulse emitted from the pulse light source through the optical fiber.
However, when imaging a particle irradiated by the light pulse that has passed through the optical fiber, speckle noise (black and white spotted background) such as shown in FIG. 9 may appear in the image. Speckle noise is caused by random interference between modes within the optical fiber, defects inside the optical fiber and distortion of the core diameter, and expansion of the core diameter due to temperature fluctuation.